Reaction Mechanism and the Role of Copper in the Photooxidation of Alcohol over Cu/Nb2O5

Loading of a small amount of copper on Nb2O5 significantly enhances the activity of alcohol photooxidation without organic solvents. Alcohol is adsorbed on the Lewis acid site (NbV) to form an alkoxide species. Photogenerated holes and electrons on Cu/Nb2O5 are trapped by the adsorbed alkoxide and Cu-II species to form the alkoxide carbon radical and Cu-I species. The formed alkoxide carbon radical is converted to a carbonyl compound and then desorbed. Finally, the reduced Cu-I sites are reoxidized by reaction with O-2. The alcohol photooxidation over Nb2O5 takes place under not only UV irradiation but also under visible light irradiation up to 450 nm, although the band gap of Nb2O5 is 390 nm (3.2 eV). DFT calculations reveal that 1) the surface donor level derived from the adsorbed alkoxide species is located in the forbidden band, 2) direct electron transition from the surface donor level to the conduction band takes place by absorbing a photon, 3) the excitation energy from surface donor level to the Nb 4d conduction band is lower than that from the O 2p valence band to Nb 4d. The kinetic study and FT/IR spectra suggest that Cu-I acts as an effective desorption site for the products. Based on these results, we conclude that copper functions as an effective redox promoter and desorption site for the product.